Snubbing is often performed to remove pipe string from an oil or gas well while the well is still live (i.e. under pressure) and avoid having to kill the well (i.e. reducing the pressure in the well). Pipe strings are formed of numerous pipe joints connected together. Unlike wireline and coiled tubing, which have a uniform outer diameter along the entire length of their run down hole, the pipe joints have collars or flanges at their ends which are used to connect the pipe joints together. These collars are larger than the outer diameter of the pipe joint. Therefore, to snub a pipe string out of a well, these collars must be accommodated while maintaining the pressure in the well bore.
Snubbing is typically done on pipe strings in conventional oil and gas wells. For SAGD operations (steam assisted gravity drainage), snubbing operations are typically not performed. Although SAGD operations commonly use pipe strings in the production wells, the SAGD process creates problems with the snubbing process. Commonly, if a pipe string or a section of a pipe string has to be removed from a SAGD well, the well is typically killed and then the pipe string is removed after the well has been killed.
One of the problems encountered with trying to snub a SAGD well is the temperature of the pipe string in a SAGD operation. SAGD operations typically use a pair of vertically spaced, horizontal well bores to collect heavy oil from a formation. The higher of the two well bores has a steam string and the lower a production string. Steam is pumped down the higher of the well bore through the steam string into the heavy oil formation where the steam heats the heavy oil, decreasing its viscosity and allowing it to flow. Gravity can then cause this heavy oil to flow downwards in the formation and into the lower horizontal well bore where it can enter the production string and be pumped up to the surface. Because the heavy oil has been heated by the steam, the steam strings and production string are subjected to elevated temperatures by the heated heavy oil.
When a SAGD well is killed, the pressure in the well is bled off and the well is cooled. However, killing a SAGD well before removing a pipe string does have disadvantages. Killing the SAGD well can result in significant downtime for the well. Once the pressure in the well has been bled of and cooled and the pipe string removed and reinstalled in the well, the formation will have to be reheated to once again get the heavy oil in the formation flowing. Steam injection will have to be restarted to the formation and the time required to get the formation up to a producing temperature can result in significant downtime.